232 Proceedings of the Royal Society of Victoria. 



The mode of observing is as follows : — The stand being placed 

 on a solid pier of stoue, brick-work, or other material, and 

 properly levelled, it is so placed that when a pendulum is placed 

 in it, one of the mirrors shall face the observer, who sits down 

 from five to seven feet away, with a good steady table or tripod 

 stand in front of him to support the coincidence apparatus, the 

 candle or lamp for illuminating the slit, a break circuit chrono- 

 meter, a telegraph key or commutator, and a portable galvanic 

 cell. He then arranges the apparatus so that he can read his 

 millimetre scale in front of the box as it is reflected by the 

 mirror on the pendulum. He now connects up his coincidence 

 apparatus with his clock or chronometer, when the electro- 

 magnet lifts the shutter every second, and an instantaneous flash 

 is seen by means of the telescope reflected from the mirror. He 

 next sets the pendulum swinging through a very small arc by 

 means of the impulse lever, when the images of the scale and 

 slit, as seen reflected from the pendulum mirror, oscillate in a 

 vertical direction over a distance magnified by both the telescope 

 and the distance of the mirror from the telescope. The fixed 

 mirror on the stand reflects a stationary image of the flash at 

 each occurrence, while the reflection from the pendulum mirror 

 occurs successively at all parts of the vertical arc over which it 

 oscillates. The moment of coincidence is when the latter appears 

 in a horizontal line with the flash reflected by the fixed mirror. 

 The time elapsed between coincidences in the same direction of 

 the pendulum's motion is the " coincidence period,"' the mean 

 value of which in twenty-four hours is what is sought, so_ as to 

 obtain the true number of vibrations made in a solar day by the 

 several pendulums of the set, from which a mean value is 

 deduced. 



In a brief description it is undesirable to describe in detail the 

 various adjustments and corrections, which are numerous ; but 

 what I have given here will afford some idea of the new set of 

 half-seconds pendulums and the method of using them. 



